Man's timeless lesson (5)
I believe the answer is:
moral
'lesson' is the definition.
(synonyms)
'man's timeless' is the wordplay.
'man' becomes 'm'.
'timeless' becomes 'oral' (I can't justify this - if you can you should believe this answer much more).
'm'+'oral'='MORAL'
(Other definitions for moral that I've seen before include "Concerned with principles of right and wrong" , "Lesson of a fable" , "Pertaining to right and wrong conduct" , "Upright; message of fable" , "Relating to principles of right and wrong" .)